The increase in studies focusing on aberrant metabolic conditions, i.e., diabetes, and those conditions which may cause metabolic alterations away from what has been described as normal, i.e., aging, necessitates the careful examination of gender differences in substrate utilization and hormone response to metabolic stress. It is therefore, the intent of this project to examine the gender differences in carbohydrate and lipid utilization during a prolonged submaximal exercise paradigm. Stable- isotopic tracers for glucose, free fatty acids, and glycerol will be used to determine substrate kinetics. In addition, substrate oxidation will be calculated from indirect calorimetry. Measurements of catecholamines, as well as insulin, will help to provide insight into any gender differences in substrate profiles which may develop during exercise. The findings from this project will add to the greater understanding of gender specific response to metabolic perturbation, and provide information to serve as a basis for using women in metabolic studies.